I don't have a denon, but I know a little about circuits. It may be overheating. Check any heat sinks and make sure there still attached to whatever they're cooling. Try placing a box fan on it and see if it runs longer. The added airflow can sometimes make up for a poor heatsink connection.
It could also be a short somewhere else in the system causing a resettable fuse to pop.
Hopefully you didn't pay much for the system.